The present invention relates generally to extrusion dies, and more particularly to apparatus for controlling material flow through the die.
Extrusion processes have long been used to produce sheets or webs of formable material, such as thermoplastic. These processes generally involve forcing a viscous material through a die typically comprising an inlet, a cavity, and an exit. As the material passes through the die, it is often necessary to influence the flow to obtain a product at the die exit having desirable characteristics.
Past methods of influencing material flow have included the use of a flow control device mounted in a channel within a die cavity, such as a restrictor bar, a flexible membrane, or another insert. Since most extrusion processes operate under high pressure, leakage of material into spaces between the insert and the channel is a significant concern. Build-up of material in the channel can prevent proper flow control and result in burning of material therein. Hence, the channel and insert must be designed and machined to tolerances that will ensure a tight sealing fit and thereby prevent leakage.
The assignee of the instant application has offered for sale a die having a flexible membrane mounted by spaced mounting members in correspondingly-shaped tapered channels in the die cavity. Bolts extend into the mounting members and are tightened to cause the members to seal against walls forming the tapered channel. End surfaces of the membrane are clamped between body members of the die to form a seal between the die and the membrane.
While this arrangement of elements is effective to accomplish flow control, some drawbacks are encountered. Specifically, the installation process is complicated and expensive. Further, machining of the tapered channels is difficult to accomplish with the required degree of precision and is not well suited (due to the channel configuration) to large-scale production techniques. In addition, re-working of such a die would likely result in a substantial amount of refitting of the flow control device in the channels. Manufacture of an aftermarket flow control device without the original tooling would also be problematic. Furthermore, clamping the end surfaces of the membrane can restrict adjustment of the membrane which, in turn, can make it more difficult to achieve a desired flow control.
According to one aspect of the present invention, a membrane assembly for an extrusion die includes a membrane having a deformable main portion and first and second mounting arms integral with the main portion and disposed on opposite sides thereof wherein the main portion includes a first face. The mounting arms extend away from the first face and the main portion, the mounting arms and a second face opposite the first face form a recess on a side of the main portion opposite the first face. A backing member is disposed in the recess between the mounting arms.
Preferably, the backing member includes a channel therein adjacent the main portion and the membrane assembly further includes an adjustment member disposed in the channel in abutment with the main portion. Also preferably, the adjustment member is serrated and includes a plurality of alternating lands and grooves wherein each land includes a substantially planar bearing surface. Still further, a movable member is preferably in contact with one of the bearing surfaces and the movable member comprises an adjustment screw which is rotatable to move the bearing surface and thereby deform the main portion. Also, the first face may be concave in shape.
Still further in accordance with the preferred embodiment, the second face includes a substantially planar central portion, first and second curved portions disposed on either side of the central portion, a first shoulder adjacent the first curved portion and a second shoulder adjacent the second curved portion. In addition, the main portion may include offset portions of differing thicknesses. Also, the backing member may include a first plurality of alternating lands and grooves and the main portion may include a second plurality of alternating thick and thin portions wherein the lands are aligned with the thick portions and the grooves are aligned with the thin portions.
In accordance with a further aspect of the present invention, an extrusion die includes a pair of body portions forming a passage therebetween and a membrane assembly is disposed in one of the body portions. The membrane assembly includes a membrane having a deformable main portion disposed in the passage and first and second mounting arms integral with the main portion and disposed on opposite sides thereof The main portion includes a first face and the mounting arms extend away from the first and the main portion. The mounting arms and a second face form a recess on a side of the main portion opposite the first face. The membrane assembly further includes a backing member disposed in the recess between the mounting arms.
Other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings and appended claims.